tty: Check tty->count instead of TTY_CLOSING in tty_reopen()

tty: Check tty->count instead of TTY_CLOSING in tty_reopen()

Although perhaps not obvious, the TTY_CLOSING bit is set when the
tty count has been decremented to 0 (which occurs while holding
tty_lock). The only other case when tty count is 0 during a re-open
is when a legacy BSD pty master has been opened in parallel but
after the pty slave, which is unsupported and returns an error.

Thus !tty->count contains the complete set of degenerate conditions
under which a tty open fails.

Reviewed-by: Alan Cox <[email protected]>
Signed-off-by: Peter Hurley <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>